The present invention generally relates to an input condition sensing circuit and more particularly to such a circuit which is configurable for sensing across a wire pair either the presence or absence of a voltage or a low or high impedance. The input condition sensing circuit finds particular applicability for use in a facility management system.
Facility management systems are used in virtually every type of large commercial structure including office buildings, hotels, manufacturing facilities, and the like. Such systems provide monitoring and control of internal environment conditions in such structures and may also, in many applications, provide monitoring of fire alarm and security systems. Facility management systems monitor and control internal environment conditions such as, for example, temperature, humidity, air quality, and air circulation.
To provide such monitoring and control, facility management systems employ sophisticated control equipment which perform closed-loop operations for maintaining the internal environment conditions of points distributed throughout a structure at certain desired set points. To that end, facility management systems generally include remote devices distributed throughout a structure which include temperature, air quality or humidity sensors which sense various parameters to be controlled and field devices such as fan motors, heaters, and dampers which are turned on and off or set to desired levels by the control equipment. The control equipment operate the field devices responsive to the parameter levels being sensed by the remote sensors and preselected set points which are preprogrammed into the control equipment. The field devices, because they are distributed throughout a structure, are generally coupled to the control equipment by relatively long cables which may include wire pairs.
In order to provide proper control, the control equipment of facility management systems must be able to discern whether field devices are operating. For example, a system may need to detect whether a certain fan is on or off. This may be accomplished by sensing if a pair of air flow detector relay contacts are open or closed or whether a fan is receiving power by detecting whether a voltage potential is present or absent at the fan power relay. For detecting such conditions, the air flow detector relay contacts and fan power relay terminals are coupled back to the control equipment by relatively long cables including a wire pair.
If an air flow detector is used, the control equipment will detect a low impedance across the wire pair indicating that the relay contacts of the flow detector are closed, and if the fan motor is not running, the control equipment will detect a high impedance across the wire pair indicating that the relay contacts are open. Because wire pairs and relay contacts impose impedance of its own, an impedance of, for example, 100,000 ohms or less may indicate closed contacts, and an impedance of 800,000 ohms or more may indicate open contacts. If the system is arranged to detect power, the control equipment will detect the presence of a voltage potential, such as 120 volts AC, across the wire pair when the fan is running and will detect the absence of a voltage potential across the wire pair if the fan is not running.
The control equipment of modern-day facility management systems generally utilize digital circuitry for processing data. Hence, with such equipment, it is necessary to provide information, such as the presence or absence of a voltage potential, or a low or high impedance in binary form. This may be achieved through the use of an input condition sensing circuit of the type to which the present invention pertains. Many condition-sensing inputs are generally required in a facility management system. In the past, different types of input condition sensing circuits have been required for sensing impedance and voltage. Hence, a given condition sensing input has been dedicated for a specific type of input condition, either a voltage condition or an impedance condition. In many applications, this can unduly limit the number and type of field devices employed. Also, if the same input condition circuit is used for sensing both voltages and impedances, there is a potential safety hazard to maintenance personnel. This results because the impedance counterpart to a voltage present condition is a high impedance (open contact) condition. If the same input condition sensing circuit is used for both applications, the circuit will provide the same binary output for both a voltage present condition and a high impedance condition. Hence, if a maintenance technician is disconnecting a wire pair having a voltage across the wires from the control equipment, this will create a contact open condition. He, therefore, will not see a change in the output binary state of the input condition sensing circuit and may become confused as to whether or not a voltage is present. This creates the potential of the maintenance technician touching both wires and receiving a potentially fatal electric shock.
The present invention overcomes the aforementioned problems by providing an input condition sensing circuit which is configurable to operate in either a first mode for sensing the presence or absence of a voltage potential or a second mode for detecting a low or high impedance. In doing so, the input condition sensing circuit of the present invention is arranged to provide different binary outputs for the presence of a voltage and for an open circuit condition. The input condition sensing circuit may be employed whenever a binary output is desired responsive to the presence or absence of a voltage or a low or high impedance condition. As will be seen hereinafter, the input condition sensing circuit of the present invention is preferably employed in a facility management system expansion module which provides a binary input interface to the facility management control equipment.